A few ideas

Juggle The Factory but instead of making the horizontal carry from left to right or right to left, carry the ball from front to back & drop the ball down behind your shoulder for a Behind the back catch.

Juggle Eriksson's Shuffle but instead of holding a ball in the hand catch & throw it vertically in a Fork. This is a shape distortion of The Box.

Hold all 3 balls in one hand & extend your arm out to the side. Throw a Triplex in towards you so that all 3 balls form a triangle in line with your body with 1 ball at the bottom & 2 at the top. Claw the lowest ball with the same hand that threw it. Then with the same hand catch the ball furthest away from you Penguin style. At the same time catch the ball closest to you behind the back with the other hand. Throw a Split multiplexbehind the back

Catch one ball in an Elbow catch (the easy way so the arm is bent) reach behind the head with the same arm then drop the ball held in that hand behind the opposite shoulder. At the same time throw the ball in the other hand up to the hand behind the head for a Behind the head catch before quickly reaching back to catch the ball that was dropped over the shoulder.